This disclosure concerns an invention relating generally to methods for obtaining and preparing specimens for microscopic analysis, and more specifically to methods of obtaining and preparing specimens for micron-scale and sub-micron-scale analysis, particularly specimens of multilayered materials, and materials upon which thin films have been deposited, implanted or otherwise incorporated (e.g., semiconductor wafers, photonic devices).
In the manufacture of many modern devices containing microscopically thin layers of different materials, and/or zones of different materials segregated on a microscopic scale, it is important to be able to study the different layers and/or zones with analytical equipment after the deposition. As examples, it is often useful to be able to microscopically analyze the structures of semiconductor microelectronic devices; magnetic thin film memory storage devices (such as read/write hard disk heads and platters); thin film based optical devices; multilayered polymeric, organic and/or biochemical based thin film devices (as used in medicine); composites of inorganic materials, organic materials and/or biological materials (such as bioMEMs, biosensors, bioarray chips, and integrated labs on chips); and other devices wherein nanoscale structures are critical to device function. Common equipment used for such analysis (hereinafter referred to as xe2x80x9cmicroanalysisxe2x80x9d) includes electron microscopes (including TEMs, Transmission Electron Microscopes, and SEMs, Scanning Electron Microscopes); spectrometers (including Raman spectrometers and Auger spectrometers); photoelectron spectrometry (XPS); Secondary Ion Mass Spectrometry (SIMS); and more recently, the atom probe microscope, as described in U.S. Pat. Nos. 5,061,850 and 5,440,124. Of course, other microanalysis equipment is available, and new equipment having different principles of operation is expected to become available over time.
Generally, microanalysis of an entire device is not feasible owing to practical constraints, and thus specimens of portions of the device are studied. Ideally, the specimen of the device under study is formed from the actual material that is intended to perform a function in the device. Accordingly, destructive testing methods are known wherein study specimens are xe2x80x9cbiopsiedxe2x80x9d from the objects being studied, and are then subjected to microanalysis. As an example, Focused Ion Beam (FIB) milling processes are often used to excise study specimens from study objects. A good background discussion of FIB processes is set forth in U.S. Pat. No. 6,042,736 to Chung. U.S. Pat. No. 6,188,072 is then of interest for its discussion of a method (allegedly described by the FEI Company of Hillsboro, Oreg., USA) of cutting a study specimen from a study object by FIB milling, with the study specimen then being removed by a micromanipulator by use of electrostatic attraction. The study specimen is then subjected to TEM microanalysis. The remainder of the patent is directed to a micromanipulator suitable for performing this operation. U.S. Pat. No. 6,188,068 to Shaapur et al. appears to describe a similar method, and the Background section of U.S. Pat. No. 5,270,552 also appears to describe similar methods for preparing study specimens using FIB milling and mechanical cutting/polishing steps.
U.S. Pat. No. 6,194,720 to Li et al. describes a method wherein a study object is milled by FIB and other processes to produce a thin cross-sectional study specimen suitable for microanalysis by a TEM. One aspect of the method involves milling a pair of parallel trenches in the top surface of the study object to define a plate-like first study region therebetween (FIGS. 2A-2C of Li et al.), and then filling in the trenches with filler material (FIG. 2D). Portions of the study object are then cut away along planes parallel to the first study region and intersecting the filled trenches (FIG. 3B), or being spaced a short distance away from the filled trenches (FIG. 3C). As a result, the study object is formed into a plate-like shape wherein the first study region defines an area of decreased thickness. The plate-like study object is then milled into a wedge-like form (FIGS. 4A and 4B) wherein the thinner side(s) of the study object define a second study region. The first and second study regions thereby define thin plate-like areas on the study object wherein the various deposited layers of the study object are displayed. A somewhat similar arrangement is described in U.S. Pat. No. 5,656,811, which is more directly devoted to methods of controlling the FIB milling process.
U.S. Pat. No. 5,270,552 describes a process wherein a study specimen is partially severed from a study object using FIB milling (with the study specimen remaining attached to the study object by a thin bridge of material), a probe is then connected to the partially-disconnected study specimen (as by xe2x80x9csolderingxe2x80x9d it thereon with FIB deposition), and then the study specimen is fully removed by cutting away the bridge with FIB milling so that the probe may carry the study specimen to a desired location for study. By using an electrically conductive probe, the voltage between the probe, study specimen, and bridge can provide a measure of whether the study specimen is intact. The probe may also serve as a support structure for further preparation of the study specimen, or for use during the study specimen""s microanalysis. Use of the process to obtain multiple study specimens from points spaced about a semiconductor wafer is illustrated. The patent additionally discusses the use of the underlying process steps to separate elements from one chip, transport them to another chip by use of the probe, and then sever the probe and xe2x80x9csolderxe2x80x9d the elements to the second chip by use of FIB deposition.
Other patents note that study specimens can be formed from a study object by use of material removal processes other than FIB processes (and any accompanying polishing or other mechanical material removal processes). U.S. Pat. No. 6,140,652 to Shlepr et al. describes the formation of study specimens from a study object for TEM microanalysis using photolithography and chemical etching processes. Trenches are etched in the study object to form a circular plug-like study specimen, which then has its base cut free from the study object by further chemical etching techniques. The study specimen can then be microanalyzed using TEM techniques.
In many instances, destructive testing (as in the foregoing methods) is undesirable because it will effectively render the study object inoperable. Thus, in some cases xe2x80x9cproxyxe2x80x9d or xe2x80x9cqualifierxe2x80x9d study objects are used: objects which are not the true study objects of interest, but which are subjected to the same processes so that they (hopefully) serve as a reasonable representation of the product generated by these processes. As an example, in the field of semiconductors, many thin film deposition systems are designed to deposit layers over an area greater than the size of a typical semiconductor wafer. Qualifier wafers are often processed alongside actual wafers so that they receive the same deposited layers as the production wafer. The qualifier wafer is then destructively tested in place of the actual wafer. However, testing of a qualifier wafer assumes that the qualifier wafer receives the same treatment as the actual wafer within the deposition system, an assumption which is not always valid because the deposited coatings may vary in time or location within the deposition system.
One significant problem encountered with all known methods is the time and expense of subsequent testing. Often, individual study specimens, once obtained in accordance with the foregoing methods, must then be individually prepared for subsequent microanalysis. This can include steps such as polishing, mounting, application of protective or other layers, situating the study specimen in a vacuum environment, and so on. Because of the disadvantages of destructive test methods, and because of the time and expense involved in the microanalysis of individual study specimens, there is a need for new methods of microanalysis which are nondestructive (or at least minimally destructive), and which are better suited for rapid processing of multiple study specimens.
The invention involves methods which are intended to at least partially solve the aforementioned problems. To give the reader a basic understanding of some of the advantageous features of the invention, following is a brief summary of preferred versions of the methods. As this is merely a summary, it should be understood that more details regarding the preferred versions may be found in the Detailed Description set forth elsewhere in this document. The claims set forth at the end of this document then define the various versions of the invention in which exclusive rights are secured.
The invention includes methods of obtaining and preparing specimens, particularly specimens of thin film materials and other materials having distinct zones of different materials arrayed on a micron or sub-micron scale (e.g., integrated circuit wafers), for study by microanalysis equipment. The invention is particularly suitable for preparing specimens for microanalysis with an atom probe, which is a preferred mode of microanalysis because it can produce three-dimensional compositional images with atomic-scale resolution. This capability of atom probes is especially attractive for studying and characterizing the small-scale structures typically found in microelectronic devices that are used, for example, in integrated electronic circuits and the read/write heads of data storage devices. Historically, atom probes have utilized a needle-shaped study specimen (or a study specimen having a needle-shaped study region formed thereon), since such a needle shape is beneficial for creating the high electric fields required for atom probe microanalysis. Where the study specimen or study region is wire-shaped, this shape readily lends itself to needle creation; otherwise, the region to be studied must be cut into a suitable needle-like shape, as by FIB milling. Planar structures like wafer-processed materials, e.g., microelectronic materials, are often difficult to cut into atom probe specimens because the structures of interest exist only in a very thin layer on the surface of the specimen that is often less than about 10 micrometers (microns) thick. However, with advances in atom probe technology, and with the advent of scanning atom probes and local electrode atom probes, it is possible to use atom probes to microanalyze specimens that raised in relation to their surroundings by as little as a few micrometers, and which are closely spaced (e.g., by no more than a few micrometers away) in relation to adjacent protrusions. For example, local electron atom probes only require a small protrusion on the specimen (a few microns high) for the local electrode to be able to locally apply the necessary extraction field to the specimen in order to effect ionization.
In a first preferred version of the invention, a study specimen is formed from a larger first study object such as an integrated circuit wafer, as by cutting the study specimen therefrom by the use of FIB milling. The study specimen will generally be a portion of the first study object item which is of key interest for microanalysis, e.g., a functional section of a semiconductor chip. The study specimen is then removed from the first study object, and is situated on a second study object such as a silicon-based wafer whereupon the study specimen is microanalyzed. Preferably, the study specimen (and perhaps several other study specimens) are also inserted within recesses in the second study object, and/or are affixed to the second study object (as by FIB deposition). The study specimen(s) can then be microanalyzed on the second study object, which can be constructed and configured to enhance the speed and ease of microanalysis; for example, the second study object may be formed of a material which promotes electrostatic attraction of the study specimen to the second study object (either by itself or with the assistance of an applied charge), thereby assisting in the placement of the study specimen on the second study object.
In a second preferred version of the invention, a study specimen is formed from a larger first study object such as a silicon-based wafer, as by cutting the study specimen therefrom by use of FIB milling. The study specimen is removed from the first study object and is situated on a second study object, with the second study object in this case generally being the item of primary interest for microanalysis rather than the first study object. The study specimen (and perhaps multiple other study specimens) can additionally be inserted within recesses formed in the second study object, and/or can be affixed to the second study object (as by FIB deposition). Where the study specimen is recessed within the second study object, it is often also useful to render the study specimen at least substantially coplanar with the second study specimen, as by the use of polishing processes. The second study object (with the study specimen thereon) is then subjected to any desired manufacturing processes, e.g., layer deposition processes, so that the study specimen and second study object both reflect the results of such processes. The study specimen may then be microanalyzed for desired information regarding the effects of the manufacturing process, preferably after removal from the second study object. Here, the second study object (and more specifically the effect of the manufacturing process on the second study object) is of primary interest for microanalysis, but the study specimen is analyzed in place of the second study object so that the second study object may be left intact, without the need to excise a portion of the second study object (as is done to the first study object in the first version of the invention). During the foregoing process, the study specimen is preferably placed on a nonfunctional portion of the second study object so that the study specimen does not interfere with the effects of the manufacturing process on the second study object; for example, where the second study object is a semiconductor chip bearing an integrated circuit, the study specimen is preferably placed on a portion of the chip which does not bear the circuit so that the circuit emerges from the manufacturing process in an operable state.
Where an atom probe is used for microanalysis of the study specimens, it will generally be useful to form study regions on the study specimens wherein protrusions are defined for generation of the desired extraction voltage. The study regions (and the protrusions therein) may be formed in the study specimens at the outset of the foregoing methods (e.g., when the study specimens are first formed), or near the end of the foregoing methods (e.g., immediately prior to microanalysis). Formation of the study regions during construction of the study specimens themselves is particularly preferred for sake of speed and efficiency, and methods are described later in this document for allowing such early formation of the study regions without leading to significant degradation in the quality of data obtained during later microanalysis.
The invention is particularly well adapted to allow sampling of objects for microanalysis while the objects are being manufactured, with minimal or no damage to the object being sampled, and with exceptionally rapid preparation of specimens for microanalysis. Further advantages, features, and objects of the invention will be apparent from the following detailed description of the invention in conjunction with the associated drawings.